This invention relates to a color image forming process and a silver halide photographic light-sensitive material utilizing a novel 2-equivalent magenta color forming coupler (also referred to herein simply as the "magenta coupler").
It is known that, upon color development of a silver halide color photographic material, an oxidized aromatic primary amine color developing agent reacts with a coupler to form an indophenol, indoaniline, indamine, azomethine, phenoxazine, phenazine or like dye, thereby forming color images. Silver halide emulsions selectively sensitive to blue, green and red light, respectively, and yellow, magenta and cyan color image-forming compounds which are in a complementary color relation to the sensitivity of the respective emulsions are employed. For example, acylacetanilide or dibenzoylmethane couplers are generally used for the formation of a yellow color image, pyrazolone, pyrazolobenzimidazole, cyanoacetophenone and indazolone couplers are generally used for the formation of magenta color images, and phenolic couplers (e.g., phenols and naphthols) are generally used for the formation of cyan color images.
In one of the most preferred embodiments of color photographic light-sensitive materials, dye image-forming couplers are added to silver halide emulsions. Couplers added to emulsions must be rendered non-diffusible (or at least diffusion-resistant).
Most conventional color-image-forming couplers are 4-equivalent couplers. That is, the development of 4 mols of silver halide as an oxidizing agent is theoretically necessary to form 1 mol of dye through the coupling reaction. On the other hand, 2-equivalent couplers are also known having an active methylene group substituted with a group (often referred to as "coupling-off group") eliminatable upon oxidative coupling of the coupler with an oxidation product of an aromatic primary amine developing agent. Such 2-equivalent couplers require the development of only 2 mols of silver halide to form 1 mol of dye. Since 2-equivalent couplers require only one-half the silver halide as compared with conventional 4-equivalent couplers to form a dye, their use enables rapid processing of light-sensitive materials due to the thinness of the light-sensitive layers, improvement of the photographic properties due to a reduction in film thickness, and results in economic advantages.
Several approaches have thus far been suggested to produce 2-equivalent 5-pyrazolone couplers (primarily for use as magenta-forming couplers). For example, the substitution of the 4-position of a pyrazolone with a thiocyano group is described in U.S. Pat. Nos. 3,214,437 and 3,253,924, with an acyloxy group is described in U.S. Pat. No. 3,311,476, with an aryloxy group is described in U.S. Pat. No. 3,419,391, with a 2-triazolyl group is described in U.S. Pat. No. 3,617,291, with an oxalic acid ester group is described in U.S. Pat. No. 3,926,631, with a sulfonamido group is described in West German Patent Application (OLS) No. 2,526,112, and with a halogen atom is described in U.S. Pat. No. 3,522,052.
However, in using these 4-position substituted pyrazolone couplers, there are disadvantages, e.g.: serious color fog may result; the reactivity of the couplers may be unsuitable; the couplers may be chemically so unstable that they are converted to materials incapable of color formation over a period of time; and the synthesis of such couplers is often difficult.
Also, it has hitherto been known to substitute the 4-position of a 5-pyrazolone with an alkylthio group, an arylthio group or a heterocyclic ring thio group, as described in U.S. Pat. No. 3,227,554. However, with many of these known thio-substituted pyrazolone compounds, the reactivity with the oxidation product of an aromatic primary amino color developing agent is unsuitable and, further, they are difficult to employ in ordinary color light-sensitive materials due to the strong photographic action of the mercapto compound produced as a result of the coupling reaction. In addition, the chemical stability of these couplers is not generally satisfactory, and the couplers change in quality resulting to a decrease in the color forming density over a period of time after coating, which is usually a fatal defect.